1. Field of the Invention
This invention relates to a switching power converting apparatus, and more particularly to switching power converting apparatus with a secondary side regulation.
2. Description of the Related Art
Referring to FIG. 1, a conventional self-excited flyback switching power converting apparatus disclosed in U.S. Pat. No. 7,835,163 converts an input voltage (Vin) to an output voltage (Vout). The conventional switching power converting apparatus utilizes a controller 12 for secondary side regulation at a secondary side of a transformer 11 to control a switching device 13 at a primary side of the transformer 11, to achieve better regulation.
Referring to FIGS. 1 and 2, a circuit configuration between a first control terminal (CTL1) and a second control terminal (CTL2) in the controller 12 may be configured as one of a resistor load connection, a short circuit and an open circuit. When the output voltage (Vout) is greater than a target voltage, the controller 12 is operable to set the circuit configuration to the resistor load connection for a time period so as to stop the switching device 13 from being conducted. Therefore, self-excited conversion from the input voltage (Vin) to the output voltage (Vout) stops, such that the output voltage (Vout) is prevented from rising. When the output voltage (Vout) is smaller than the target voltage, the controller 12 is operable to set the circuit configuration to the short circuit for a time period so as to cause the switching device 13 to conduct. Therefore, the self-excited conversion starts for raising the output voltage (Vout). Otherwise, the controller 12 is operable to set the circuit configuration to the open circuit so as to avoid interfering with operation of the conventional switching power converting apparatus.
The following are some of the drawbacks of the conventional switching power converting apparatus:
1. It is hard for a designer to determine a resistance value between the first and second control terminals (CTL1, CTL2) of the controller 12 with the circuit configuration being the resistor load connection.
2. The resistance value between the first and second control terminals (CTL1, CTL2) of the controller 12 may vary in fabrication process of the controller 12, and may drift along with variation in temperature, etc.